A couple of options for you.
One... you could get an actual antler stain, however if the antlers are slightly faded there's a chance it may add too much color for them. Most commercial antler stains come in a bottle that you could stain 50 sets of antlers with, but it can also be added in coats. If you would like just a small amount I could probably set you up with some to try.
Two...coffee stain them. Take about a cup of 'Dark Roast' ground beans and mix just enough water to make a paste. Preferably outside, and wearing gloves, take about a tablespoon in your hand and liberally rub over the antler. This will bring back color and can easily be done again to attain the color depthness your looking for. Its messy, but it works quite well and Ive done this on several chalk/white moose antlers and it turned out great.
Three... I use Watco 'Medium Walnut' Floor Wax on most of my antlers as a finisher. Its the liquid form and more like a stain consistency then a 'wax'. A small amount on a rag or paper towel enhances the color slightly and can be added in several layers to attain a darker/stain quality. It also rehydrates the antlers and protects them, just ensure to wipe off the excess after it dries to keep the antlers from keeping a real shiny look.
Hope this helped and best of luck.
Joel-BRT